I need to know?

take one of your villagers to study the cactus... check him/her as a doctor (healing in the details box) first..

also, anyone villager can heal another villager.. just that the doctor is more guaranteed of success the first time.. also take him around to the various plants/flowers...

And even children can gain healing skills, so I'll usually try to find a child that wasn't already marked as an "apprentice" and use him or her to heal a sick villager. That'll make it easier for the kid to become a doctor later.

Villagers that have at least a "Trainee Doctor" level will occasionally go to "study" the cactus rather than just be "curious" about it.